There are many applications where using a component to failure is unacceptable, and thus the component must be replaced when the risk of failure is too high. The decision of when to retire a component is a tradeoff between at least the cost of replacement and the risk of failure should the part continue to be used.
Component failure is preceded by deterioration in the condition of the component. Deterioration of a component's condition is caused by the development and growth of flaws in the component. Flaws for metals may include cracks, microcracks, inclusions, residual stress variations, microstructure variations, mechanical damage such as dents and scratches, corrosion pits, and machining effects. Flaws for composites might include fiber damage, bridging, impact damage, disbands, and delaminations. Flaws may originate during manufacture or develop once the component is in service. While in service the component may be exposed to operating conditions that lead to the development and/or further growth of the flaw. Different types of components may be more sensitive to different types of loads. Operating conditions that may affect the condition of a component may include temperature, temperature variation (e.g., freeze-thaw cycles), acceleration, vibration, voltage, pressure, rotational speed, mechanical stress, static loading, dynamic loading, impact events, and any other physical process that contributes to the development and/or growth of component flaws.
In many applications a component is in use intermittently and thus the operating conditions may not be persistent in time. Accordingly, the in-service time of a component may be measured in effective usage cycles, rather than in time directly. For example, an airplane component may be exposed to adverse operating conditions principally during each take off and landing cycles (or ground-air-ground, “GAG”, cycles). The operating environment while the aircraft is grounded or cruising may have significantly less contribution to flaw growth than the operating conditions during takeoff and landing. Accordingly, a suitable in-service time unit may be takeoff/land cycles. Though, other suitable measures of in-service time may be used.
Safe life models have been used to predict the life of components. These models consider the operating conditions that cause damage to a component and estimate the intensity of these conditions while the component is in service. Assuming an initial flaw site, safe life models predict the growth of the flaw as the component is exposed to worst case operating conditions. Component failure may be defined, for example, by a point in the growth of a flaw in the component at which the component may no longer serve its intended purpose. The component may be replaced when the service time of the component reaches some fraction of the service time at which the component is predicted by the safe life models to fail (e.g. 50%).
Periodic inspection of components may also be used to detect flaws. The inspection may not only look for the presence of flaws but also to characterize the flaw with one or more features. For example, a crack in a component may be characterized by the crack's length. Component flaw growth models may then be used to predict, for example, the likelihood the flaw will lead to component failure by a future time. Plot 100, shown in FIG. 1, sketches a curve 101 representing the probability of failure within a time, Δt. A damage tolerance limit 103 is selected based on an acceptable probability of failure 105.
Because failure is probabilistic, inspections are traditionally scheduled periodically so that a flaw can be detected early in its growth cycle, well before it is likely to develop to the point of causing component failure. Different inspection technologies will be capable of detecting flaws at different points in their growth cycle and therefore the inspection interval depends upon the type of inspection being performed and its expected detection performance at the location of interest.